From German Patent 813,154, for example, it is known to provide a double worm extruder having two worms rotatable in the same sense of rotation and with which a plastifiable material is subjected to mixing, shear and pressure plastification purposes. The material may be a thermoplastic material which is to be extruded through an extruder head at one end of the extruder to be delivered to an injector at that end for injection molding of the material, or can force the material through a die to form a shaped strand thereof.
In the double worm extruder of this patent, along the worms and at the same axial locations therealong, kneading units are provided, each of which can consist of three or more angularly or circumferentially offset polygonyl disks which at two diametrically opposite locations have diameters which correspond to the diameters of the worm housing so that the disks effectively form a seal with the housing or are closely juxtaposed therewith.
Double worm extruders or compounding machines of this type are used to knead and mix synthetic resin material primarily and the worm constructions are generally matched to the type of materials which are to be subjected to mixing, plastification and shear. The material can be more or less viscous, plastic, pulverulent or granular and the worms can be dimensioned accordingly.
For adjacent operation of the double worm extruder and for high quality extruded products, it is important that the worms insure good and thorough mixing with a minimum of thermal decomposition of the material during its advance through the worm housing.
The worms of the double worm extruder described in German Patent 813,154 have, as noted, kneading units along their length which are intended to improve the mixing material in the worm housing.
Each of these kneading units comprises a multiplicity of axially adjacent but circumferentially and angularly offset polygonyl disks, i.e. disks having a plurality of sides, all of which may be arcuate, analogous to the configuration of a cam.
These disks operate on the material by forming constrictions between themselves and with the cylindrical wall of the worm housing, through which the material can be forced.
The polygonal disks have diametrically opposite sealing surfaces which are provided at the largest transverse dimensions of these disks and are so closely juxtaposed with the wall of the cylinder that they form sealing surfaces with the inner wall of the worm housing. In these regions, therefore, the material cannot be forced past the disk and thus the material must flow around these areas over nonsealing surfaces of the polygon disks.
Since the kneading units are provided adjacent one another at the same axial locations along the worms, the sealing surfaces of the polygon disks of the two kneading units can practically meet between the worms so that here as well the material cannot be forced past the sealing surfaces but must flow around the disks.
The kneading units of the two worms have comparatively small free flow cross sections through which the material can pass. As a consequence, shear spikes can develop, i.e. regions of extremely high shear, with concomitant peaks in heating and the heating can be in excess of that which is desirable for heat sensitive materials so that thermal decomposition can occur at least to a limited extent.